Renal Function and Renal Function Tests

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Introduction

The kidneys are among the most important organs in the human body. They continuously filter the blood, remove metabolic waste products, regulate water and electrolyte balance, maintain acid–base equilibrium, control blood pressure, and support red blood cell production. Healthy kidneys are essential for maintaining the body's internal environment and ensuring that organs function properly.

Despite their importance, kidney diseases can develop silently. A person may have significant kidney damage without experiencing obvious symptoms during the early stages. For this reason, renal function tests are essential tools for identifying kidney problems, monitoring existing kidney disease, evaluating treatment responses, and assessing the safety of certain medications.

Renal function refers to the ability of the kidneys to perform their normal physiological activities. Renal function tests, commonly called RFTs, are laboratory investigations used to evaluate different aspects of kidney health. These tests include measurements of serum creatinine, blood urea, estimated glomerular filtration rate, electrolytes, and urine abnormalities. Depending on the clinical situation, additional investigations may include urine albumin-to-creatinine ratio, cystatin C, urinalysis, and imaging studies.

Understanding renal function and renal function tests is important for medical students, healthcare professionals, laboratory personnel, and anyone interested in maintaining kidney health.

This comprehensive article explains kidney anatomy and physiology, the meaning of renal function, the major renal function tests, normal laboratory ranges, the interpretation of abnormal results, common kidney diseases, and practical approaches to prevention and treatment.

1. What Is Renal Function?

Renal function describes how effectively the kidneys perform their essential tasks. The kidneys are two bean-shaped organs located toward the back of the abdominal cavity, on either side of the vertebral column.

Each kidney contains approximately one million nephrons, although the number varies between individuals. The nephron is the structural and functional unit of the kidney. It filters blood, reabsorbs useful substances, and removes unwanted materials through urine formation.

The kidneys receive a substantial proportion of the heart's cardiac output. This continuous blood supply allows them to regulate the composition of blood and eliminate waste products efficiently.

Normal renal function involves several interconnected processes.

A. Filtration of Blood

The kidneys filter blood through tiny structures called glomeruli. Water and small dissolved substances pass from the blood into the filtrate, while blood cells and most proteins remain in the circulation.

This filtration process removes substances such as urea, creatinine, and other waste products.

B. Removal of Metabolic Waste

The body continuously produces waste products during normal metabolism. These substances must be eliminated to prevent their accumulation.

The kidneys remove waste products including:

• Urea, which is produced during protein metabolism.
• Creatinine, which is generated from muscle creatine metabolism.
• Uric acid, which results from purine metabolism.
• Certain drugs and their metabolites.
• Excess ions and other water-soluble substances.

When renal function decreases, these substances may accumulate in the blood.

C. Regulation of Water Balance

The kidneys regulate how much water the body retains or eliminates. When the body needs to conserve water, the kidneys produce more concentrated urine. When excess water is present, they can produce more dilute urine.

This mechanism helps maintain blood volume, blood pressure, and normal cellular function.

D. Regulation of Electrolytes

Electrolytes are electrically charged substances that are essential for normal physiological activities.

The kidneys regulate the levels of sodium, potassium, chloride, calcium, phosphate, and bicarbonate.

Abnormal electrolyte concentrations can affect muscle function, nerve transmission, heart rhythm, and other important processes.

E. Maintenance of Acid–Base Balance

Normal blood pH is maintained within a narrow range. The kidneys contribute by excreting hydrogen ions and regulating bicarbonate, an important blood buffer.

When kidney function deteriorates, acid may accumulate in the body, producing metabolic acidosis.

F. Regulation of Blood Pressure

The kidneys regulate blood pressure through sodium and water balance and the renin–angiotensin–aldosterone system.

When kidney function is impaired, the body may retain sodium and water. This can contribute to hypertension and fluid overload.

G. Production of Erythropoietin

The kidneys produce erythropoietin, a hormone that stimulates red blood cell production in the bone marrow.

Chronic kidney disease may reduce erythropoietin production, leading to anemia.

H. Activation of Vitamin D

The kidneys convert vitamin D into its biologically active form, calcitriol. This process supports calcium absorption, phosphate regulation, and bone health.

Advanced kidney disease may disturb mineral metabolism and increase the risk of bone disorders.

2. What Are Renal Function Tests?

Renal function tests are laboratory investigations used to assess kidney filtration, kidney-related biochemical abnormalities, and evidence of kidney damage.

The term RFT does not refer to one universal panel. The tests included may vary between laboratories and healthcare facilities.

A typical renal assessment may include:

• Serum creatinine.
• Blood urea or blood urea nitrogen.
• Estimated glomerular filtration rate.
• Serum sodium.
• Serum potassium.
• Serum chloride.
• Serum bicarbonate.
• Urinalysis.
• Urine albumin-to-creatinine ratio.

Additional tests may be ordered when clinically indicated.

Examples include cystatin C, measured creatinine clearance, serum calcium, phosphate, uric acid, complete blood count, and kidney imaging.

It is important to understand that no single test provides a complete picture of kidney health. Doctors interpret the results together with symptoms, medical history, physical examination, medication use, and previous laboratory reports.

3. Why Are Renal Function Tests Performed?

Renal function tests are performed for several important reasons.

A. Detection of Kidney Disease

These tests can identify abnormalities before symptoms become obvious. Early detection is especially important in people with diabetes, hypertension, cardiovascular disease, or a family history of kidney disease.

B. Diagnosis of Acute Kidney Injury

Acute kidney injury is a sudden decline in kidney function that may develop over hours or days.

Causes include severe dehydration, blood loss, sepsis, urinary obstruction, and certain medications.

Serial measurements of serum creatinine and urine output help identify and monitor this condition.

C. Monitoring Chronic Kidney Disease

Chronic kidney disease develops when abnormalities of kidney structure or function persist for at least three months.

Renal function tests help determine the severity of kidney impairment and monitor whether the condition is stable or progressing.

D. Monitoring Diabetes and Hypertension

Diabetes and hypertension are two leading causes of chronic kidney disease.

Regular assessment of estimated glomerular filtration rate and urine albumin is important because kidney damage may occur even when a person feels well.

E. Evaluating Medication Safety

Some medications can affect kidney function or require dose adjustment when kidney function is reduced.

Examples include certain antibiotics, antivirals, chemotherapy agents, and other drugs that are eliminated through the kidneys.

Renal function assessment may also be required before some imaging procedures involving contrast agents.

F. Evaluating Fluid and Electrolyte Disorders

Kidney disease may cause abnormal sodium, potassium, bicarbonate, and water levels.

Testing helps identify these disturbances and guide appropriate treatment.

G. Assessment Before Surgery

Renal function may be evaluated before major surgery, particularly when a patient has risk factors for kidney injury or when the planned procedure involves medicines or treatments that may affect the kidneys.

4. Important Tests Included in Renal Function Assessment

4.1 Serum Creatinine

Serum creatinine is one of the most commonly used blood tests for assessing kidney filtration.

Creatinine is produced mainly from the breakdown of creatine in skeletal muscle. It enters the bloodstream and is eliminated predominantly through the kidneys.

When kidney filtration decreases, serum creatinine often rises.

Typical adult reference range: approximately 0.6–1.3 mg/dL, although laboratory ranges vary and may differ according to sex, age, muscle mass, and testing method.

Causes of Increased Serum Creatinine

Increased serum creatinine may occur in:

• Acute kidney injury.
• Chronic kidney disease.
• Dehydration or reduced kidney perfusion.
• Urinary tract obstruction.
• Severe systemic illness.
• Certain medication-related effects.
• Increased creatinine production in some circumstances.

A high creatinine result does not automatically establish chronic kidney disease. Acute changes, medications, hydration, and other clinical factors must be considered.

Causes of Low Serum Creatinine

Low creatinine may be associated with:

• Low muscle mass.
• Frailty.
• Malnutrition.
• Certain muscle-wasting conditions.
• Pregnancy-related changes in filtration.

A low creatinine concentration does not necessarily indicate exceptionally healthy kidneys. In people with low muscle mass, creatinine may underestimate the severity of kidney dysfunction.

Limitations of Serum Creatinine

Serum creatinine is useful but imperfect. It may remain within the laboratory reference range during early kidney disease.

Creatinine is also affected by muscle mass, dietary intake, physical activity, and some medications.

For these reasons, estimated glomerular filtration rate is generally more useful than serum creatinine alone when evaluating filtration.

4.2 Blood Urea and Blood Urea Nitrogen

Urea is a waste product formed in the liver when the body processes nitrogen-containing substances, especially proteins.

The kidneys eliminate urea through urine. When kidney function decreases, blood urea may increase.

Laboratories may report either total blood urea or blood urea nitrogen, abbreviated BUN. These are related measurements but are not numerically identical.

Typical adult BUN reference range: approximately 7–20 mg/dL.

Typical adult blood urea range: approximately 15–45 mg/dL, depending on the laboratory and reporting method.

Always use the reference range provided by the laboratory performing the test.

Causes of Increased Blood Urea

Increased blood urea or BUN may occur in:

• Acute kidney injury.
• Chronic kidney disease.
• Dehydration.
• Reduced blood flow to the kidneys.
• Gastrointestinal bleeding.
• High protein intake.
• Increased tissue breakdown.
• Certain severe illnesses.

Causes of Decreased Blood Urea

Lower values may occur in:

• Severe liver dysfunction.
• Low protein intake.
• Malnutrition.
• Overhydration.
• Pregnancy.

Difference Between Urea and Creatinine

Urea and creatinine are both waste products, but they behave differently.

Urea is strongly influenced by protein intake, hydration, liver function, and tissue breakdown. Creatinine is more closely related to muscle metabolism and kidney filtration, although it also has limitations.

Therefore, a raised urea level with relatively stable creatinine may suggest dehydration or another nonrenal influence, but the pattern must be interpreted in context.

4.3 Estimated Glomerular Filtration Rate

Estimated glomerular filtration rate, or eGFR, is one of the most important measurements used to assess kidney filtration.

The glomerular filtration rate describes the volume of fluid filtered by the glomeruli per unit of time. Because direct measurement is difficult in routine practice, eGFR is usually calculated from blood test results using validated equations.

Modern adult equations commonly use serum creatinine, age, and sex. In selected circumstances, cystatin C or a combined creatinine–cystatin C equation may provide a more accurate estimate.

eGFR is generally reported in milliliters per minute per 1.73 square meters of body surface area.

eGFR Categories

eGFR category	Result	General interpretation
G1	90 or higher	Normal or high filtration
G2	60–89	Mildly decreased filtration
G3a	45–59	Mild to moderately decreased
G3b	30–44	Moderately to severely decreased
G4	15–29	Severely decreased
G5	Below 15	Kidney failure range

These categories are commonly used in chronic kidney disease classification.

An eGFR of 90 or higher does not rule out kidney disease if other markers of kidney damage are present. Similarly, an eGFR of 60–89 does not by itself establish chronic kidney disease.

For chronic kidney disease to be diagnosed, abnormalities generally need to persist for at least three months, unless other evidence establishes the diagnosis.

Why eGFR Is Important

eGFR helps clinicians:

• Identify reduced kidney filtration.
• Classify chronic kidney disease.
• Monitor changes over time.
• Assess the need for further investigations.
• Guide dosing of selected medications.
• Help determine when specialist evaluation is appropriate.

Limitations of eGFR

eGFR estimates are less reliable when serum creatinine is changing rapidly, such as during evolving acute kidney injury.

Accuracy may also be reduced in people with unusual muscle mass, severe malnutrition, amputation, or certain other conditions.

When the estimate is uncertain and the result would change clinical decisions, cystatin C or a measured filtration test may be considered.

4.4 Urine Albumin-to-Creatinine Ratio

Urine albumin-to-creatinine ratio, abbreviated UACR, measures the amount of albumin in urine relative to urine creatinine.

Albumin is a protein normally retained in the bloodstream. Damage to the kidney's filtration barrier may allow albumin to pass into urine.

UACR is especially important because albuminuria can indicate kidney damage even when eGFR is normal.

A spot urine sample, preferably collected as a first-morning specimen when practical, is commonly used.

UACR Categories

Category	UACR	Interpretation
A1	Below 30 mg/g	Normal to mildly increased
A2	30–300 mg/g	Moderately increased
A3	Above 300 mg/g	Severely increased

These categories refer to urine albumin measured relative to creatinine. Laboratories may also report results in mg/mmol.

Causes of Increased Urine Albumin

Increased urine albumin may occur in:

• Diabetic kidney disease.
• Hypertensive kidney damage.
• Glomerulonephritis.
• Other chronic kidney diseases.
• Certain cardiovascular and systemic disorders.

Albumin excretion can temporarily increase after strenuous exercise, during fever, with urinary tract infection, or during other acute illnesses.

Therefore, an abnormal result may need to be repeated under appropriate conditions.

Why UACR Matters

UACR helps detect early kidney damage and provides information about the risk of kidney disease progression and cardiovascular complications.

In many patients, measuring both eGFR and UACR gives a more complete assessment than measuring either alone.

4.5 Urinalysis

Urinalysis is an examination of urine that may include physical, chemical, and microscopic assessment.

It is useful for identifying abnormalities that suggest kidney disease, urinary tract infection, stones, or other urinary disorders.

Physical Examination

Physical examination may assess:

• Color.
• Clarity.
• Odor, when relevant.
• Specific gravity.

Normal urine is generally pale yellow to amber, but color varies with hydration, diet, medicines, and other factors.

Dark urine may reflect concentrated urine, blood, bilirubin, or other substances. Color alone cannot establish a diagnosis.

Chemical Examination

A urine dipstick may test for:

• Protein.
• Blood.
• Glucose.
• Ketones.
• Leukocyte esterase.
• Nitrites.
• pH.
• Specific gravity.

Protein or blood may indicate kidney or urinary tract disease, but these findings have several possible causes.

Glucose may occur in diabetes or certain kidney tubular disorders. Leukocyte esterase and nitrites may support a diagnosis of urinary tract infection when interpreted alongside symptoms and other findings.

Microscopic Examination

Microscopic examination may identify:

• Red blood cells.
• White blood cells.
• Casts.
• Crystals.
• Epithelial cells.
• Bacteria.

Certain casts, particularly red blood cell casts, may suggest inflammation within the kidney's filtering structures. White blood cell casts can occur in some renal inflammatory or infectious conditions.

Crystals may be seen in urine with or without kidney stones.

The interpretation depends on the complete clinical picture.

4.6 Serum Electrolytes

Electrolytes are important components of renal function assessment because the kidneys regulate their concentrations.

Sodium

Sodium is essential for extracellular fluid balance, nerve transmission, and muscle function.

Typical adult serum sodium range: 135–145 mmol/L.

Low sodium is called hyponatremia, while high sodium is called hypernatremia.

Kidney disease can contribute to sodium disturbances, but they may also result from medicines, hormonal disorders, gastrointestinal fluid losses, and excessive or inadequate water intake.

Potassium

Potassium is essential for normal muscle contraction and electrical activity in the heart.

Typical adult serum potassium range: approximately 3.5–5.0 mmol/L.

Low potassium is called hypokalemia. High potassium is called hyperkalemia.

Kidney dysfunction can cause hyperkalemia because the kidneys normally help eliminate excess potassium.

Severe hyperkalemia can produce dangerous heart rhythm disturbances and requires urgent medical assessment.

A falsely elevated result may occur when blood cells are damaged during sample collection or processing. Clinicians may repeat the test when appropriate, but a potentially dangerous result should not be ignored while awaiting confirmation.

Chloride

Chloride helps maintain fluid balance and acid–base equilibrium.

Typical adult serum chloride range: approximately 98–106 mmol/L.

Abnormal chloride levels may occur with dehydration, gastrointestinal losses, kidney disorders, and acid–base disturbances.

Bicarbonate

Bicarbonate is an important component of the body's buffering system.

Typical adult serum bicarbonate range: approximately 22–29 mmol/L.

Reduced bicarbonate may suggest metabolic acidosis, which can occur in advanced kidney disease, certain tubular disorders, severe diarrhea, or other conditions.

In kidney disease, metabolic acidosis may contribute to muscle breakdown and bone complications if it persists untreated.

Reference ranges vary between laboratories, and results should always be interpreted using the reporting laboratory's standards.

4.7 Serum Calcium and Phosphate

The kidneys help regulate calcium and phosphate balance and activate vitamin D.

In chronic kidney disease, phosphate retention and altered vitamin D metabolism may contribute to disturbances in calcium regulation and parathyroid hormone levels.

These changes are collectively associated with chronic kidney disease–mineral and bone disorder.

Serum calcium and phosphate testing may be particularly useful in patients with moderate or advanced chronic kidney disease.

4.8 Serum Uric Acid

Uric acid is produced during the breakdown of purines. The kidneys contribute substantially to its elimination.

High serum uric acid can occur in people with reduced kidney function, but it can also result from diet, genetic factors, medicines, and increased uric acid production.

Elevated uric acid is associated with gout and uric acid kidney stones in some individuals.

However, serum uric acid is not a stand-alone test of kidney filtration. A normal result does not rule out kidney disease, and an elevated result does not prove that kidney damage is present.

4.9 Cystatin C

Cystatin C is a small protein produced by most nucleated cells. It is filtered by the kidneys and can be used to estimate glomerular filtration.

Unlike creatinine, cystatin C is less dependent on muscle mass. It may therefore be helpful when creatinine-based eGFR is suspected to be inaccurate.

Cystatin C can be affected by factors such as inflammation, thyroid dysfunction, smoking, and corticosteroid use. It is not perfect, but combining cystatin C with creatinine can improve filtration estimates in selected patients.

4.10 Creatinine Clearance

Creatinine clearance estimates the volume of plasma cleared of creatinine per unit of time.

It may be calculated using a timed urine collection, often over 24 hours, together with a blood creatinine measurement.

Creatinine clearance can be useful in selected situations, but it has limitations. Incomplete urine collection and tubular secretion of creatinine may affect the result.

For routine assessment of most adults, validated eGFR equations are generally preferred.

4.11 Kidney Imaging

Imaging is not itself a laboratory test, but it is an important part of evaluating kidney disease.

Ultrasound may identify:

• Kidney size and structure.
• Hydronephrosis.
• Certain kidney stones.
• Cysts or masses.
• Some structural abnormalities.

Other imaging methods may be used when clinically indicated.

Imaging can help identify obstruction or structural disease, but a normal ultrasound does not rule out many forms of kidney disease.

5. How to Prepare for Renal Function Tests

Preparation depends on which tests have been ordered.

Many routine blood tests for renal function do not require fasting. However, a healthcare professional may request fasting if other tests are being performed at the same time.

Patients should follow the instructions given by their laboratory or clinician.

Important preparation points include:

• Drink water normally unless instructed otherwise.
• Avoid deliberate dehydration before testing.
• Tell the clinician about prescription medicines, over-the-counter medicines, and supplements.
• Mention recent strenuous exercise if it may affect interpretation.
• Follow instructions for timed urine collection carefully.
• Inform the clinician about recent vomiting, diarrhea, fever, or other acute illness.

Do not stop prescribed medicines without medical advice. Some medications affect creatinine or potassium, and clinicians need to know about them to interpret results correctly.

For a 24-hour urine collection, the laboratory should provide specific instructions. Incorrect collection can make the result unreliable.

6. Normal Renal Function Test Results

There is no single universal normal RFT panel. Reference intervals vary according to the laboratory, assay method, age, sex, pregnancy status, and other factors.

The following values are approximate examples for adults and are not substitutes for the reference intervals on an actual laboratory report.

Test	Approximate adult reference range
Serum creatinine	About 0.6–1.3 mg/dL
Blood urea nitrogen	About 7–20 mg/dL
Blood urea	About 15–45 mg/dL
Serum sodium	135–145 mmol/L
Serum potassium	About 3.5–5.0 mmol/L
Serum chloride	About 98–106 mmol/L
Serum bicarbonate	About 22–29 mmol/L
eGFR	Usually 90 or higher in healthy younger adults
Urine albumin-to-creatinine ratio	Below 30 mg/g

A result within a reference range does not always exclude disease. Conversely, a mildly abnormal result does not always indicate serious illness.

The most important considerations are the overall pattern, symptoms, previous results, and whether abnormalities persist.

7. Understanding Abnormal Renal Function Test Results

Abnormal RFT results must be interpreted systematically.

A. Raised Creatinine and Reduced eGFR

This pattern suggests reduced kidney filtration, but the cause may be acute or chronic.

Possible explanations include dehydration, acute kidney injury, chronic kidney disease, obstruction, and medication effects.

Previous laboratory results are extremely valuable. A creatinine concentration that has risen rapidly may require urgent evaluation even if the absolute value is not dramatically high.

B. Raised Urea With Relatively Stable Creatinine

This pattern may occur in dehydration, gastrointestinal bleeding, high protein intake, or increased tissue breakdown.

It may also occur in kidney disease. Further assessment is necessary rather than relying on the urea result alone.

C. Albumin in Urine

Albuminuria may indicate damage to the kidney's filtering barrier.

Persistent albuminuria can be present even when eGFR remains normal. This is why urine albumin testing is particularly important in people with diabetes or hypertension.

D. Blood in Urine

Blood in urine may arise from kidney inflammation, kidney stones, urinary tract infection, tumors, or other urinary tract disorders.

Menstruation, strenuous exercise, and sample contamination can also affect results.

Persistent microscopic blood in urine requires appropriate assessment, particularly when accompanied by proteinuria or reduced kidney function.

E. High Potassium

Hyperkalemia may result from reduced kidney potassium excretion, certain medicines, hormonal abnormalities, or tissue breakdown.

Because severe hyperkalemia can affect cardiac electrical activity, substantially elevated potassium requires prompt medical evaluation.

F. Low Bicarbonate

Low bicarbonate may indicate metabolic acidosis. In kidney disease, this can result from reduced acid excretion or impaired bicarbonate handling.

Other causes must also be considered.

G. Multiple Abnormalities

A combination of reduced eGFR, increased urine albumin, abnormal potassium, and low bicarbonate may indicate more advanced kidney dysfunction.

The severity depends on the degree of abnormality, how rapidly it developed, the patient's symptoms, and associated conditions.

8. Acute Kidney Injury

Acute kidney injury, abbreviated AKI, is a sudden decline in kidney function. It can develop over hours or days and may be reversible if the underlying cause is identified and treated promptly.

AKI is commonly classified according to the main mechanism responsible.

A. Prerenal Causes

Prerenal AKI results from inadequate blood flow to the kidneys without initial structural damage to the kidney tissue.

Causes include:

• Severe dehydration.
• Significant blood loss.
• Low blood pressure.
• Severe heart failure.
• Sepsis-related circulatory disturbances.

If blood flow is restored promptly, kidney function may improve. Prolonged inadequate perfusion can progress to structural kidney injury.

B. Intrinsic Renal Causes

Intrinsic AKI results from damage within the kidneys themselves.

Causes include:

• Acute tubular injury.
• Glomerulonephritis.
• Interstitial nephritis.
• Kidney infections involving the tissue.
• Certain toxins and medication-related injuries.

Treatment depends on the specific disease process.

C. Postrenal Causes

Postrenal AKI occurs when urine flow is obstructed.

Possible causes include:

• Enlarged prostate.
• Kidney or urinary tract stones.
• Tumors causing obstruction.
• Urethral obstruction.
• Certain structural abnormalities.

Prompt relief of obstruction can be essential for recovery.

Symptoms of Acute Kidney Injury

Some patients have few symptoms, while others develop:

• Reduced urine output.
• Swelling of the legs or face.
• Shortness of breath.
• Nausea or vomiting.
• Fatigue.
• Confusion in severe cases.

Urine output may remain normal in some cases of AKI, so normal urination does not exclude it.

Diagnosis and Treatment

Diagnosis generally relies on changes in serum creatinine, urine output, clinical assessment, and investigations directed at the suspected cause.

Treatment may include correcting dehydration, treating infection, stopping or adjusting harmful medicines under medical supervision, relieving urinary obstruction, and managing electrolyte disturbances.

Severe cases may require hospital admission or dialysis.

9. Chronic Kidney Disease

Chronic kidney disease, or CKD, is defined by abnormalities of kidney structure or function that persist for at least three months and have implications for health.

CKD can progress slowly over years. Many people do not notice symptoms until the disease becomes advanced.

Common Causes

The major causes include:

• Diabetes mellitus.
• Hypertension.
• Glomerular diseases.
• Inherited kidney disorders, such as polycystic kidney disease.
• Recurrent kidney injury.
• Chronic urinary tract obstruction.
• Certain systemic diseases.
• Some long-term medication-related injuries.

Stages of Chronic Kidney Disease

CKD is classified using both eGFR and albuminuria.

The G1–G5 categories describe filtration, while A1–A3 categories describe urine albumin.

A patient with an eGFR of 55 and normal albumin excretion has a different risk profile from a patient with the same eGFR and severe albuminuria.

Risk assessment therefore requires both measurements when possible.

Symptoms of Chronic Kidney Disease

Early CKD may be asymptomatic. As kidney function deteriorates, possible symptoms include:

• Fatigue.
• Reduced appetite.
• Nausea.
• Itching.
• Swelling of the feet or ankles.
• Changes in urination.
• Muscle cramps.
• Sleep disturbances.
• Difficulty concentrating.
• Breathlessness caused by fluid overload or anemia.

These symptoms are not specific to kidney disease, and some patients with advanced CKD have relatively few symptoms.

Complications

CKD may cause or contribute to:

• Hypertension.
• Cardiovascular disease.
• Anemia.
• Hyperkalemia.
• Metabolic acidosis.
• Fluid overload.
• Mineral and bone disorders.
• Malnutrition.
• Accumulation of metabolic waste products.

Early diagnosis and appropriate management can reduce complications and slow progression in many patients.

10. Other Important Kidney Diseases

A. Glomerulonephritis

Glomerulonephritis refers to inflammation or injury involving the glomeruli.

It may develop after infections, in association with autoimmune disease, or as part of other conditions.

Possible findings include blood in urine, proteinuria, swelling, hypertension, and reduced kidney function.

Evaluation may require urine studies, blood tests, imaging, and occasionally a kidney biopsy.

B. Diabetic Kidney Disease

Diabetes can damage the kidney's small blood vessels and filtration structures.

Early disease may be detected through increased urine albumin, while later stages may show a declining eGFR.

Good glucose control, blood pressure management, and appropriate kidney-protective medicines can reduce the risk of progression.

C. Hypertensive Kidney Disease

Long-standing high blood pressure can damage renal blood vessels and tissue.

Kidney disease may also worsen hypertension, creating a cycle in which each condition contributes to the other.

Regular blood pressure monitoring, appropriate medication, and kidney testing are important for affected patients.

D. Polycystic Kidney Disease

Polycystic kidney disease is an inherited disorder characterized by fluid-filled cysts in the kidneys.

Depending on the type and severity, it may cause enlarged kidneys, hypertension, pain, blood in urine, kidney stones, and progressive kidney dysfunction.

Family history and imaging can help establish the diagnosis.

E. Kidney Stones

Kidney stones form when minerals and other substances crystallize in urine.

They may cause severe flank pain, blood in urine, nausea, or urinary obstruction.

Small stones may pass spontaneously, while larger or obstructing stones may require medical or procedural treatment.

F. Urinary Tract Infection

A urinary tract infection can involve the bladder or, in more serious cases, the kidneys.

Kidney infection may cause fever, chills, flank pain, nausea, and urinary symptoms.

Urinalysis and urine culture may help establish the diagnosis. Prompt treatment is important, particularly when infection is severe or obstruction is suspected.

11. Relationship Between Renal Function and Blood Pressure

The kidneys and cardiovascular system are closely connected.

The kidneys regulate blood volume by controlling sodium and water excretion. They also release renin, which participates in the renin–angiotensin–aldosterone system.

When this system is activated inappropriately, blood vessels may constrict and the body may retain sodium and water.

This can increase blood pressure.

Hypertension, in turn, damages small renal blood vessels and may accelerate kidney disease.

For patients with CKD, blood pressure management is therefore a major part of kidney protection. The target blood pressure depends on individual circumstances, measurement methods, age, symptoms, and other medical conditions.

12. Relationship Between Renal Function and Diabetes

Diabetes is a major cause of chronic kidney disease.

Persistently elevated blood glucose can damage the glomeruli and other kidney structures. Early changes may cause albumin to leak into the urine before eGFR declines.

For this reason, kidney screening in diabetes usually includes both:

• eGFR, calculated from serum creatinine.
• Urine albumin-to-creatinine ratio.

The frequency of testing depends on the type and duration of diabetes, previous results, and clinical guidance.

Good glucose management, blood pressure control, and appropriate use of kidney-protective medicines may reduce the risk of kidney failure.

13. Medicines That May Affect Kidney Function

Many medicines are safe when used appropriately, but some can cause kidney injury or require dose adjustment in people with reduced renal function.

Examples include:

• Nonsteroidal anti-inflammatory drugs, such as ibuprofen and naproxen.
• Certain antibiotics.
• Some antiviral medicines.
• Selected chemotherapy agents.
• Some immunosuppressive drugs.
• Certain diuretics, depending on the clinical situation.
• Medicines that can increase potassium, particularly in susceptible patients.

The risk depends on the medicine, dose, duration, hydration status, other medications, and the patient's underlying health.

NSAIDs may reduce blood flow to the kidneys, especially in people with dehydration, CKD, heart failure, or certain medication combinations.

However, many medicines that influence kidney measurements are also beneficial and should not be stopped automatically. For example, ACE inhibitors and angiotensin receptor blockers can protect the kidneys in many patients with albuminuria, although they may alter creatinine and potassium. Clinicians monitor these changes to balance benefits and risks.

Patients should not discontinue prescribed treatment without professional advice.

14. Kidney Function and Contrast Agents

Some imaging procedures use contrast agents to improve visibility of blood vessels and organs.

The risk associated with contrast depends on the type of contrast agent, the patient's kidney function, the procedure, and other clinical factors.

For iodinated contrast used in CT imaging, the greatest concern is generally in patients with acute kidney injury or substantially reduced eGFR.

The risk should be assessed individually. Necessary imaging should not automatically be withheld simply because a patient has kidney disease.

Appropriate precautions may include reviewing recent kidney function, assessing hydration, and considering alternatives when clinically suitable.

15. Kidney Function in Older Adults

Kidney filtration often decreases with age, although the degree varies considerably between individuals.

Older adults may also have reduced muscle mass, making serum creatinine appear deceptively normal despite impaired filtration.

For this reason, eGFR is particularly useful in older patients.

Medication review is also important because many drugs are eliminated through the kidneys. Dose adjustments may be necessary even when the patient does not report kidney-related symptoms.

However, age alone should not be used to dismiss abnormal results. The interpretation must consider the patient's overall condition and evidence of kidney damage.

16. Kidney Function During Pregnancy

Pregnancy causes physiological changes in kidney blood flow and filtration. As a result, serum creatinine is usually lower during pregnancy than in nonpregnant adults.

A creatinine concentration that might appear normal according to a general adult reference range can be concerning during pregnancy.

Kidney disease during pregnancy may also be associated with hypertension, proteinuria, and complications affecting both mother and baby.

Assessment may involve serum creatinine, urine protein measurements, blood pressure monitoring, and additional tests depending on the clinical situation.

Estimated GFR equations designed for nonpregnant adults are not considered reliable for routine assessment during pregnancy. Pregnancy-specific clinical interpretation is necessary.

17. Renal Function Tests in Children

Kidney function tests in children require age-appropriate interpretation.

Normal creatinine levels vary with age, body size, and muscle development. Adult reference ranges should not automatically be applied to infants or young children.

Pediatric eGFR calculations may require height and a formula validated for children.

Causes of kidney disease in children may include congenital abnormalities, inherited disorders, glomerular diseases, infections, and other conditions.

Persistent proteinuria, blood in urine, high blood pressure, abnormal growth, or recurrent urinary infections may require further assessment.

18. When Should Renal Function Tests Be Repeated?

The timing of repeat testing depends on the reason for testing and the severity of any abnormality.

A healthy person undergoing routine screening may require testing at a different interval from a patient with known CKD or a recent episode of AKI.

Repeat testing may be necessary:

• After an unexpected abnormal result.
• Following an episode of dehydration or severe illness.
• After starting or changing certain medicines.
• When monitoring diabetes or hypertension.
• During follow-up of established kidney disease.
• After treatment of urinary obstruction.
• When potassium or other electrolytes are abnormal.

A single abnormal result should not automatically be labeled chronic kidney disease. Persistence over time and the clinical circumstances are important.

However, suspected acute kidney injury or dangerous electrolyte abnormalities may require immediate assessment rather than waiting for routine follow-up.